Lithographic apparatus and positioning apparatus

ABSTRACT

A lithographic apparatus comprises a substrate table to hold a substrate, a reference structure and a measurement system to measure a position of the substrate table with respect to the reference structure. The measurement system comprises a first measurement system to measure a position of the substrate table with respect to an intermediate structure and a second measurement system to measure a position of the intermediate structure with respect to the reference structure. The intermediate structure may be connected or connectable to a drive mechanism to drive the substrate table. A distance between the substrate table and the intermediate structure, and a distance between the intermediate structure and the reference structure may be small which results in a highly accurate position measurement.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 11/109,860, which was filed on Apr. 20, 2005. The contents ofwhich is incorporated herein by reference.

BACKGROUND

1. Field of the Invention

The present invention relates to a lithographic apparatus and apositioning apparatus.

2. Description of the Related Art

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In such a case, a patterning device, which isalternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.including part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Conventional lithographicapparatus include so-called steppers, in which each target portion isirradiated by exposing an entire pattern onto the target portion atonce, and so-called scanners, in which each target portion is irradiatedby scanning the pattern through a radiation beam in a given direction(the “scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

The lithographic apparatus according to the state of the art may includea substrate table which is constructed to hold a substrate. Whetherusage is made of the so-called stepper or the so-called scanner, in eachcase it is required that the substrate table is movable over arelatively large distance, e.g. to irradiate different part of thesubstrate, to perform measurements on different parts of the substrate,to exchange the substrate, etc. Due to high accuracy which is requiredfor the pattern which is to be brought onto the substrate in any way, ahigh positioning accuracy of the substrate is required. To achieve anaccurate positioning, The lithographic apparatus includes a measurementsystem to measure a position of the substrate table, e.g. with respectto a reference structure such as the metrology frame which may alsoserve as a reference for a projection system of the lithographicapparatus. Due to their inherent high accuracy's, interferometers may beused in the measurement system to measure the position of the substratetable. For this purpose, several types of interferometers are known, anexample being described in Agilent's Laser and Optics User's Manual (p/n05517-90045). The measurement system may detect the position of thesubstrate table in e.g. a two dimensional plane, however commonlyseveral other degrees of freedom are measured too, including e.g. arotation of the substrate table, a height with respect to the twodimensional plane, etc. Due to the large range of movement which isrequired for the substrate table as has been described above, a lengthof a measurement beam of an interferometer comprised in the measurementsystem may very significantly depending on a position of the substratetable. If the substrate table is at its current position relativelyclose to a particular interferometer, then a beam length of thatparticular interferometer will be relatively short, while if thesubstrate table is at another instance at a position relatively remotefrom that particular interferometer, then the beam length of themeasurement beam of that particular interferometer might be relativelylong. In a practical implementation, an interferometer beam length mayextend up to a distance in an order of magnitude of e.g. 0,5 meters. Thepossibly large and varying interferometer beam length results in adeterioration of a measurement accuracy of the interferometer. As known,a read out of an interferometer is dependent on a wavelength of theinterferometer beam. This wavelength in turn depends on a plurality ofphysical quantities, amongst which a temperature, an atmosphericpressure of a gas through which the interferometer beam travels, etc.Due to all kind of disturbance factors, such as a movement of thesubstrate table, movements of any other movable part of the lithographicapparatus, generation of heat, airflow, etc, variations in such physicalparameters which have an influence on the wavelength of theinterferometer beam may occur, thus affecting or locally affecting awavelength of the interferometer beam. Therefor, an accuracy of themeasurement system is limited by these factors, which may result in alimitation in an accuracy to position the substrate table, which may inturn lead to a limitation in an accuracy in applying the pattern on thesubstrate.

SUMMARY

It is desirable to provide an accurate measurement system to measure aposition of a substrate table or any other movable part of alithographic apparatus or of any other positioning apparatus.

According to an embodiment of the invention, there is provided alithographic apparatus comprising a substrate table constructed to holda substrate, a reference structure, and a measurement system to measurea position of the substrate table with respect to the referencestructure, wherein the measurement system comprises a first measurementsystem to measure a position of the substrate table with respect to anintermediate structure, and a second measurement system to measure aposition of the intermediate structure with respect to the referencestructure.

In another embodiment of the invention, there is provided a positioningapparatus comprising a substrate table constructed to hold a substrate,a reference structure, and a measurement system to measure a position ofthe substrate table with respect to the reference structure, wherein themeasurement system comprises a first measurement system to measure aposition of the substrate table with respect to an intermediatestructure, and a second measurement system to measure a position of theintermediate structure with respect to the reference structure.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1A depicts a lithographic apparatus according to an embodiment ofthe invention;

FIG. 1B shows a stage of the lithographic apparatus of FIG. 1A;

FIG. 2 highly schematically depicts a part of a lithographic apparatusaccording to an embodiment of the invention;

FIG. 3 depicts a perspective view of the embodiment according to FIG. 2;

FIG. 4 depicts another perspective view of the embodiment according toFIG. 2;

FIG. 5 depicts a perspective view of a part of a lithographic apparatusaccording to another embodiment of the invention; and

FIG. 6 highly schematically depicts a view of a part of a lithographicapparatus according to still another embodiment of the invention.

DETAILED DESCRIPTION

FIG. 1A schematically depicts a lithographic apparatus according to oneembodiment of the invention. The apparatus includes an illuminationsystem (illuminator) IL configured to condition a radiation beam B (e.g.UV radiation or any other suitable radiation), a mask support structure(e.g. a mask table) MT constructed to support a patterning device (e.g.a mask) MA and connected to a first positioning device PM configured toaccurately position the patterning device in accordance with certainparameters. The apparatus also includes a substrate table (e.g. a wafertable) WT or “substrate support” constructed to hold a substrate (e.g. aresist-coated wafer) W and connected to a second positioning device PWconfigured to accurately position the substrate in accordance withcertain parameters. The apparatus further includes a projection system(e.g. a refractive projection lens system) PS configured to project apattern imparted to the radiation beam B by patterning device MA onto atarget portion C (e.g. including one or more dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The mask support structure supports, i.e. bears the weight of, thepatterning device. It holds the patterning device in a manner thatdepends on the orientation of the patterning device, the design of thelithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The mask support structure can use mechanical, vacuum, electrostatic orother clamping techniques to hold the patterning device. The masksupport structure may be a frame or a table, for example, which may befixed or movable as required. The mask support structure may ensure thatthe patterning device is at a desired position, for example with respectto the projection system. Any use of the terms “reticle” or “mask”herein may be considered synonymous with the more general term“patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section so as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables or “substrate supports” (and/or two or more masktables or “mask supports”). In such “multiple stage” machines theadditional tables or supports may be used in parallel, or preparatorysteps may be carried out on one or more tables or supports while one ormore other tables or supports are being used for exposure.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g. water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system. Immersion techniques can beused to increase the numerical aperture of projection systems. The term“immersion” as used herein does not mean that a structure, such as asubstrate, must be submerged in liquid, but rather only means that aliquid is located between the projection system and the substrate duringexposure.

Referring to FIG. 1A, the illuminator IL receives a radiation beam froma radiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDincluding, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may include an adjuster AD configured to adjust theangular intensity distribution of the radiation beam. Generally, atleast the outer and/or inner radial extent (commonly referred to asσ-outer and σ-inner, respectively) of the intensity distribution in apupil plane of the illuminator can be adjusted. In addition, theilluminator IL may include various other components, such as anintegrator IN and a condenser CO. The illuminator may be used tocondition the radiation beam, to have a desired uniformity and intensitydistribution in its cross-section.

The radiation beam B is incident on the patterning device (e.g., maskMA), which is held on the mask support structure (e.g., mask table MT),and is patterned by the patterning device. Having traversed the mask MA,the radiation beam B passes through the projection system PS, whichfocuses the beam onto a target portion C of the substrate W. With theaid of the second positioning device PW and position sensor IF (e.g. aninterferometric device, linear encoder or capacitive sensor), thesubstrate table WT can be moved accurately, e.g. so as to positiondifferent target portions C in the path of the radiation beam B.Similarly, the first positioning device PM and another position sensor(which is not explicitly depicted in FIG. 1A) can be used to accuratelyposition the mask MA with respect to the path of the radiation beam B,e.g. after mechanical retrieval from a mask library, or during a scan.In general, movement of the mask table MT may be realized with the aidof a long-stroke module (coarse positioning) and a short-stroke module(fine positioning), which form part of the first positioning device PM.Similarly, movement of the substrate table WT or “substrate support” maybe realized using a long-stroke module and a short-stroke module, whichform part of the second positioner PW. In the case of a stepper (asopposed to a scanner) the mask table MT may be connected to ashort-stroke actuator only, or may be fixed. Mask MA and substrate W maybe aligned using mask alignment marks M1, M2 and substrate alignmentmarks P1, P2. Although the substrate alignment marks as illustratedoccupy dedicated target portions, they may be located in spaces betweentarget portions (these are known as scribe-lane alignment marks).Similarly, in situations in which more than one die is provided on themask MA, the mask alignment marks may be located between the dies.

FIG. 1B shows a substrate stage St (also called substrate chuck) for thelithographic apparatus according to FIG. 1A. The stage St comprises thenon-stationary parts of the second positioner PW, a mirror block MB, andthe substrate table WT mounted to the mirror block MB. In this examplethe mirror block MB is provided with interferometer-mirrors which arearranged for cooperation with interferometers for measuring the positionof the mirror block MB.

The second positioner is arranged for positioning the mirror block MBand the substrate table WT. The second positioner PW comprises the shortstroke module (which is provided with a short stroke motor ShM) and thelong stroke module (which is provided with a long stroke motor LoM).

The long stroke motor LoM comprises a stationary part LMS that can bemounted to a stationary frame or a balance mass (not shown) and anon-stationary part LMM that is displaceable relative to the stationarypart. The short stroke motor ShM comprises a first non-stationary partSMS (that may be mounted to the non-stationary part LMM of the longstroke motor) and a second non-stationary part SMM (that may be mountedto the mirror block MB).

It should be noted that the mask table MT and the first positioner PM(see FIG. 1A) may have a similar structure as depicted in FIG. 1B.

A so-called dual stage machine may be equipped with two or more stagesas described. Each stage can be provided with an object table (such asthe substrate table WT). In such an arrangement, a preparatory step suchas the measurement of a height map of the substrate disposed on one ofthe object tables can be performed in parallel with the exposure of thesubstrate disposed on another object table. In order to expose asubstrate that previously has been measured, the stages may changeposition from the measurement location to the exposure location (andvice versa). As an alternative, the object tables can be moved from onestage to another.

The apparatus depicted in FIG. 1A could be used in at least one of thefollowing modes:

1. In step mode, the mask table MT or “mask support” and the substratetable WT or “substrate support” are kept essentially stationary, whilean entire pattern imparted to the radiation beam is projected onto atarget portion C at one time (i.e. a single static exposure). Thesubstrate table WT or “substrate support” is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the mask table MT or “mask support” and the substratetable WT or “substrate support” are scanned synchronously while apattern imparted to the radiation beam is projected onto a targetportion C (i.e. a single dynamic exposure). The velocity and directionof the substrate table WT or “substrate support” relative to the masktable MT or “mask support” may be determined by the (de-)magnificationand image reversal characteristics of the projection system PS. In scanmode, the maximum size of the exposure field limits the width (in thenon-scanning direction) of the target portion in a single dynamicexposure, whereas the length of the scanning motion determines theheight (in the scanning direction) of the target portion.

3. In another mode, the mask table MT or “mask support” is keptessentially stationary holding a programmable patterning device, and thesubstrate table WT or “substrate support” is moved or scanned while apattern imparted to the radiation beam is projected onto a targetportion C. In this mode, generally a pulsed radiation source is employedand the programmable patterning device is updated as required after eachmovement of the substrate table WT or “substrate support” or in betweensuccessive radiation pulses during a scan. This mode of operation can bereadily applied to maskless lithography that utilizes programmablepatterning device, such as a programmable mirror array of a type asreferred to above.

Combinations and/or variations on the above-described modes of use orentirely different modes of use may also be employed.

FIG. 2 highly schematically depicts a side view of the substrate tableWT, the projection system PS and a reference frame or referencestructure REF in this example comprising e.g. a metrology frame. Theprojection system PS may be mounted to the reference frame REF. Thestage St may e.g. comprise a mirror block, e.g. comprising a body madeof a glass which is provided with reflective coatings on at least a partof a surface thereof. In some cases the substrate table is provided withsuch mirrors. This facilitates distance measurements withinterferometers, as in such embodiment the substrate table functions asa mirror, thus making additional parts superfluous, thus preventingadditional tolerances which could be caused thereby. As the view of FIG.2 is a highly schematic side view, it does not depict that the referencestructure may comprise a 3-dimensional structure, as will be explainedin more detail below. FIG. 2 further shows an intermediate structure IS(also referred to in this document as intermediate frame or frame),which in this embodiment extends between the substrate table WT and thereference structure REF, leaving a small opening between the substratetable WT and the intermediate structure IS as well as a small openingbetween the intermediate structure IS and the reference frame REF. Afirst measurement system (which is not shown in detail in FIG. 2 butwill be explained in more detail with reference to FIGS. 3 and 4) isshown to measure a position of the substrate table WT with respect tothe intermediate structure IS. Further, FIG. 2 shows a secondmeasurement system SMS to measure a position of the intermediatestructure IS with respect to the reference structure REF. Of course, itis also possible that the first measurement system measures a positionof the intermediate structure with respect to the substrate table and/orthat the second measurement system measures a position of the referencestructure with respect to the intermediate structure. As by the firstand second measurement system, a position of the substrate table WT withrespect to the intermediate structure, and a position of theintermediate structure with respect to the reference structure is known,a position of the substrate table WT with respect to the referencestructure REF is derivable therefrom. Further, in this embodiment adistance between the substrate table WT and the intermediate structureis short, leading to measurement beams of the first measurement system,schematically indicated in FIG. 2 by a line between the substrate tableand the intermediate structure, which are short. Similarly, a distancebetween the intermediate structure and the reference structure is shortleading to measurement beams between the intermediate structure and thereference structure, the measurement beams being schematically indicatedin FIG. 2 by a line between the intermediate structure and the referencestructure, which are short. Thus, wavelength induced measurement errorsas might occur in the measurement system according to the state of theart, may be prevented to a large extent as an optical path which themeasurement beam is required to travel between the respective parts, issubstantially shortened in the embodiment as described here. A furtheradvantage of the relative shortness of the measurement beams is thatalignment errors are less likely to occur and a beam shift may be verysmall, because of the relative shortness of an optical path to betraveled by the respective beam or beams.

The principle as illustrated above with reference to FIG. 2 will now beexplained in more detail with reference to FIG. 3 and FIG. 4.

FIG. 3 shows the substrate table WT, and the intermediate structure IS.The reference structure has not been drawn for clarity reasons, howeverthe skilled person will understand that the reference structure may belocated over a top surface of the intermediate structure, a surface ofthe reference structure facing the intermediate structure beingsubstantially parallel to a plane defined by the axis X and Y as definedin FIG. 3. FIG. 3 further shows an X-beam which may be comprised in adrive mechanism to move the substrate table along first axis indicatedhere as the Y-axis. The X-beam may therefor interface with a structurewhich extends along the Y-axis (not shown), the X-beam being movablewith respect to such structure by any type of actuator, such as a linearmotor. The actuator and the X-beam XB may form a first drive mechanismto move the substrate table along the first axis (i.e. the Y axis). Theintermediate structure is connected or connectable to the X-beam, thusin general wording to the first drive mechanism, the intermediatestructure thus following a movement of the substrate table along thefirst axis. Thus, if the substrate table is moved by the first drivemechanism along the Y axis, the intermediate structure will follow thismovement, as the intermediate structure is connected or connectable tothe first drive mechanism, in this example e.g. connected to the X-beamXB. An advantage is that a distance between the substrate table and theintermediate structure does not or not significantly change when movingthe substrate table along the first axis. Also, a distance between theintermediate structure and the reference structure does not changesignificantly, as the surface of the reference structure extends abovethe intermediate structure. Thus, the advantages of the invention asdescribed above remain present when moving the substrate table along theY-axis as the intermediate structure is able to follow or substantiallyfollow such movement, thus hardly or not affecting a distance betweenthe substrate table and the intermediate structure, as well as adistance between the intermediate structure and the reference structure.The substrate table may be connectable to the drive mechanism by aclamping or other releasable mechanism, allowing the substrate table tobe separated from the drive mechanism by releasing the clampingmechanism or other releasable mechanism, thus enabling the substratetable to be handled independently of the drive mechanism e.g. by anotherdrive mechanism, such as may be the case in a so called dual stagelithographic apparatus.

The lithographic apparatus may further comprise a second drive mechanismto move the substrate table along the X-axis, also indicated here assecond axis. Thus, when the substrate table WT is to be moved along theX-axis, the second drive mechanism (not shown) moves the substrate tableWT with respect to in this embodiment the X-beam XB. When the substratetable WT is moved by the second drive mechanism along the X-axis, adistance between the substrate table WT and the intermediate structureIS remains substantially constant, as a surface of the intermediatestructure facing the substrate table WT extends substantially parallelto the X-axis. Summarizing, a movement of the substrate table along theY-axis will be followed by the intermediate structure IS, thus not oronly insignificantly affecting a distance between the substrate tableand the intermediate structure as well as a distance between theintermediate structure and the reference structure, while a displacementof the substrate table WT along the X-axis leads to a displacement ofthe substrate table WT with respect to the intermediate structure ISalong the X-axis, as the intermediate structure does not orsubstantially not follow such movement in this embodiment, thus notaffecting a distance between the substrate table WT and the intermediatestructure. Also, in the case of the movement of the substrate tablealong the X-axis, a distance between the intermediate structure and thereference structure is not changed, as the intermediate structure inthis embodiment does not move in that case with respect to the referencestructure. Thus, the embodiment as shown in FIG. 3 makes it possible tomove the substrate table WT in both X and Y directions, whilesubstantially maintaining a same distance between the substrate table WTand the intermediate structure as well as between the intermediatestructure and the reference structure. In the embodiment shown in FIG.3, the first measurement system FMS generates a plurality of first(optical) measurement beams between the intermediate structure and thesubstrate table, these measurement beams being in a directionsubstantially parallel to the first axis, thus the Y-axis. Likewise, thesecond measurement system SMS may generate a plurality of second(optical) measurement beams between the intermediate structure and thereference structure, the second measurement beams being in a directionperpendicular to the Y-axis such as in this embodiment substantiallyperpendicular to the plane defined by the X and Y axes. A more detaileddescription of the first and second measurement system will followbelow. It is noted that, due to the effect that the distances betweenthe substrate table and the intermediate structure and between theintermediate structure and the reference structure stay substantiallyunchanged despite movements of the substrate table along the X-axisand/or Y-axis, a length of the measurement beams of the first and secondmeasurement systems does not or not substantially change, thus the beamlengths of the measurement beams of the first and second measurementsystems staying at there short length, substantially independently of aposition or position movement of the substrate table WT. Therefore, anyeffect which would deteriorate an accuracy of the position measurementsby the first a second measurement systems, is hardly present, as themeasurement beams keep their relatively short lengths over a wide rangeof movement of the substrate table. With the current state of the art,it may be impossible to keep these short lengths with large movements ofthe substrate table.

Further, with the current state of the art, at multiple sides of thesubstrate table a mirror is needed, while with the present inventiononly a mirror at one side of the substrate table may be needed. Thepresent invention therefore reduces cost of—and gives design freedom ofthe substrate table.

It is noted that in the embodiment described with reference to FIG. 3,the second drive mechanism is arranged to move the substrate table withrespect the first drive mechanism, however also other embodiments arepossible, the second drive mechanism may e.g. move the substrate tablewith respect to a same reference as the first drive mechanism. Similarlyas described with reference to the first drive mechanism, the seconddrive mechanism may also comprise any type of suitable actuator, such asa linear motor, etc. Further, it is noted that in an alternativeembodiment, the second measurement system SMS may also comprise secondmeasurement beams which are substantially parallel to the second axis;providing same or similar advantages as the embodiments as describedhere. It is noted that the arrows as depicted in the symbolsrepresenting sensors of the first and second measurement systems FMS,SMS, do not indicate a direction of the measurement beam. In stead thesesensors indicate a direction along which the respective sensor providesposition information, thus a direction along which position informationis provided by the respective position sensing device. The first andsecond measurement systems will now be described in more detail withreference to FIG. 4.

As depicted in FIG. 4, the first measurement system comprises aplurality of interferometers and a plurality of encoders. Theinterferometers as well as the encoders comprise a measurement beamwhich extends substantially parallel to the first axis, thus the Y-axis.By such a combination of interferometers and encoders, it is possible toperform a position measurement of the substrate table WT (which has beenleft away in FIG. 4 for clarity reasons) and the intermediate structure,without affecting a beam length when moving the substrate tablesubstantially parallel to the X-axis, the movement of the substratetable WT being relative to the intermediate structure IS. In theembodiment described here, the first measurement system comprises afirst interferometer IF 1, second interferometers IF2A, IF2B, and athird interferometer IF3, further, the first measurement systemcomprises a first encoder Enc1, a second encoder Enc2, and a thirdencoder Enc3. For illustrative purposes, as already remarked abovearrows originating at the respective interferometers and encoders inFIG. 4 indicate a direction of sensitivity of the respectiveinterferometer or encoder. Also, a physical appearance of the respectiveinterferometer and encoder may differ from the highly schematicrepresentation as depicted in FIG. 4. For example, it is possible thatmeasurement beams for the interferometers resp. encoders are deliveredby fibre delivery, thus optical fibres being e.g. comprised in theintermediate structure to guide the respective measurement beams to theappropriate locations of the respective interferometers and encoders.

In the embodiment as depicted in FIG. 4, the first interferometer IF1and the second interferometers IF2A, IF2B, are separated from each otherin a direction parallel to the X-axis, the third interferometer IF3being separated from the first and second interferometers in a directionperpendicular to the X- and Y-axis. Depending on a position of thesubstrate table with respect to the intermediate structure, either thefirst interferometer IF1 in combination with second interferometer IF2Amay be applied, or a combination of the first interferometer IF1 and thesecond interferometer IF2B may be applied for position measurement. Alarge range of movement along the X-axis may be coped with by the secondinterferometers as an appropriate one of the second interferometers isselected for measurement. In the embodiment as described with referenceto FIGS. 3 and 4, it is desired to obtain a position of a center of thesubstrate table, in this example a middle point of a top surface of thesubstrate table. True to the relatively large dimensions of thesubstrate table, and thus the relative long ‘arm’ between this centerand the encoders and interferometers of the first measurement system,any tolerance on a measurement by the first measurement system, inparticular a tolerance in a measurement of a rotation of the substratetable around the X-axis, leads to relatively strong errors in apositions of the middle point of the substrate table. Therefore, adistance between the interferometer IF 1 and IF3 has been chosen to belarge, as the rotation around the X-axis is derived from an output valueof the first and third interferometers. For a same reason, a distancebetween the first and second interferometers is comparably large, asfrom a read out of these interferometers, a rotation of the substratetable around the Z-axis is derived. Due to the long arm of the middlepoint of the substrate table towards the first measurement system, ahigh accuracy of these rotations is required, which is provided by therelatively large distances between the interferometers as mentioned. Thefirst and second encoder Enc1, Enc2, may comprise a scale to measure aposition in a direction substantially perpendicular to the X- andY-axis. The scale may e.g. be connected to the substrate table WT, ormay e.g. be comprised in a surface at a side of the substrate tablewhich faces the intermediate structure. Likewise, the third encoder Enc3may comprise a scale to measure a position in a direction substantiallyparallel to the second axis, i.e. the X-axis. It is advantageous toconnect the scales to the substrate table, or to integrated the scalesinto the substrate table WT, by using an e.g. reflecting side surfacethereof which faces the intermediate structure, thus making it possibleto implement the scales in a simple way, as the substrate table may beequipped with one of more reflecting side surfaces already for usage asa mirror by the interferometers IF1, IF2A, IF2B and IF3. Below, anoverview is provided of formulas for calculating a position of thesubstrate table (indicated here a mirror block), with respect to theintermediate structure (indicated here as a frame):Rx_mirrorb1=(IF3−IF1)/(dist_if3_if1)Ry_mirrorb1=(Enc2−Enc1)/(dist_Enc2_Enc1)Rz_mirrorb1=(IF2x−IF1)/(dist_if2x_if1) (IF2x can be IF2a or IF2b,depending on the x position of the mirrorblock)X_mirrorb1=Enc3+Rz_mirrorb1*(Ywaferstage−Y0) Y0=distance between thecenter of the chuck and the side of the chuck)Y_mirrorb1=IF1+Rx_mirrorb1*Dist_IF1_focus,ZcomponentZ_mirrorb1=(Enc1+Enc2)/2+Rx_mirrorb1*(Ywaferstage−Y0)

wherein Rx, Ry, Rz represent a rotation with respect to the X-, Y- andZ-axis and where dist_A_B represents a distance between A and B, wherein this expression A and B may be replaced by any interferometer orencoder. Further, the term Zcomponent refers to a component or adistance in a direction parallel to the Z-axis as shown in FIGS. 3 and4, and the term chuck refers to the substrate table.

Further, it is remarked that the first and second encoders are separatedfrom each other in a direction parallel to the Y-axis while the thirdencoder is separated from the first and second encoders in a directionparallel to the Z-axis. The Ry is determined from measurements by thefirst and second encoder. A distance between the encoders may be smallerthan a distance between the interferometers, because most of themeasurements are close to the Y-axis of the projection system.Therefore, a rotation around the Y-axis (Ry) with such a short arm willhave less influence. Thus, a required Ry accuracy for compensation ofthe arm is relatively small. It is noted that the term separated in thecontext of e.g. the encoders or the interferometers are separated may beunderstood as referring to a physical distance between the measurementbeams thereof, while this wording does not say anything about a physicalpositioning of a remainder of the interferometer or encoder in question.Also, the wording ‘are separated in a direction . . . .’ Does notexclude a separation in any other direction, thus e.g. the wording‘interferometers IF1 and IF3 are separated in a direction along theZ-axis’ does not exclude any separation along any other axis, thus thebeams of these interferometers are in this example not necessarilylocated purely vertical with respect to each other.

FIG. 4 further shows the second measurement system SMS which comprises afourth interferometer IF4, a fifth interferometer IF5, and a sixthinterferometer IF6. Further, the second measurement systems SMScomprises a fourth encoder Enc4, a fifth encoder Enc5, and a sixthencoder Enc6. In an advantageous embodiment, the fourth and fifthinterferometers IF4, IF5, are separated from each other in a directionparallel to the second axis. The sixth interferometer may be separatedfrom the fourth and fifth interferometers in a direction parallel to thefirst axis. The fifth and sixth encoders Enc5, Enc6, may comprise ascale to measure a position in a direction substantially parallel to theY-axis, while the fourth encoder Enc4 may comprise a scale to measure aposition in a direction substantially to the second axis. The fifth andsixth encoders Enc5, Enc6, may be separated from each other in adirection parallel to the second axis. With this combination of encodersand interferometers comprised in the second measurement system, aposition of the intermediate structure with regard to the referencestructure may be calculated as follows:Rx_frame=(IF6−IF5)/(dist_if6_if5)Ry_frame=(IF5−IF4)/(dist_if5_if4)Rz_frame=(Enc6−Enc5)/(dist_Enc6_Enc5)X_frame=Enc4+Rz_frame*Dist_Enc4_side_of_the_mirrorb1,YcomponentY_frame=(Enc6+Enc5)/2+Rx_frame*(dist_Enc6/Enc5_IF1,Zcomponent)Z_frame=(IF5+IF4)/2+Rx_frame*Dist_IF4/IF5_side_of_the_mirrorb1,Xcomponent

wherein Rx_frame, Ry_frame, etc represent a rotation of the frame, i.e.a rotation of the intermediate structure around the X-axis, the Y-axis,etc., while X_frame, Y_frame etc. represent a position of the frame,i.e. the intermediate structure along the X-axis, Y-axis, etc.

To prevent and/or to dampen a vibration of the intermediate structure,the intermediate structure may be connected e.g. at an extreme thereofto the first drive mechanism (or to any other suitable part) by a leafspring or any other resilient or flexible member.

To obtain an accurate relationship between a measurement by the firstmeasurement system FMS and a measurement by the second measurementsystem SMS, the intermediate structure may comprise a material having ahigh thermal stability such as an invar material, and it preferablycomprises a high stiffness. To achieve such high stiffness, theintermediate structure preferably comprises a rectangular hollow ormassive bar.

A further advantageous application of the intermediate structure will bedescribed with reference to FIG. 5. FIG. 5 depicts the substrate tableWT, intermediate structure IS, first measurement system FMS and secondmeasurement system SMS, similar or identical to the ones described withreference to the previous figures. Further, FIG. 5 depicts the X-beam XBas described above. Further, the projection system PS is highlyschematically indicated. Between the projection system PS (or moreprecisely between a down stream lens of the projection system PS) andthe substrate held by the substrate table WT, a fluid supply system LSmay be positioned to provide an immersion fluid (a liquid or a gas) tofill a space between the down stream projection element or projectionlens and the substrate. Although immersion lithography provides a numberof advantages in terms of resolution capability and optical imaging, adisadvantage is that a replacement of the substrate requires acomplicated closing mechanism to prevent immersion fluid in the fluidsupply system LS from flowing away when the substrate is exchanged.According to an aspect of the invention, the intermediate structure ISmay by applied to close the fluid supply system, thus preventing fluidfrom flowing out when e.g. replacing or moving away the substrate heldby the substrate table WT. Therefore, the lithographic apparatus may bearranged to move to the substrate table WT plus the intermediatestructure along the Y-axis in a direction to the left in the plane ofdrawing of FIG. 5, thus causing the projection system and fluid supplysystem to face the intermediate structure, in particular an area Athereof. The surface of the area A of the intermediate structure is nowable to close the fluid supply system thus preventing immersion liquidfrom flowing away. In this situation, it is possible to perform apressure measurement relating to the pressure of the immersion fluid(immersion liquid or immersion gas) in the fluid supply system, as thepressure may be derived from a displacement of the intermediatestructure in this position, thus when the area A is applied as a closureplate for closing the fluid supply system. The higher a pressure of theimmersion liquid, the more a displacement of the intermediate structuremay be observed, which may e.g. be detected by the second measurementsystem SMS. As the second measurement system provides a position of theintermediate structure in six degrees of freedom, as outlined above,information about the pressure profile may be derived from the read outof the second measurement system to, as a rotation of the intermediatestructure may be a result of e.g. a pressure difference observed by theintermediate structure IS at different parts of the area A. Thus, asample pressure measurement device may be implemented to measure apressure and/or a pressure profile of the immersion fluid in the liquidsupply system, also indicated as immersion hood. The pressuremeasurement device may comprise, in addition or instead of the pressuremeasurement as described above, a plurality of sensors, such as pressuresensors, arranged in or on the intermediate structure, in particular inor on the surface A thereof. Thus, a pressure of the immersion fluid maybe easily derived from a reading of these sensors. The sensors maycomprise sensors to provide a pressure profile.

Although marked in FIG. 5 by a circle, the area A may of coarse form anintegral part of the surface of the intermediate structure or may haveany other suitable shape (such as rectangular, ellipse shaped, etc.).The area A may have a size which is substantially equal to a size of theliquid supply system or to a size of substrate, however the area A mayalso have a larger size which provides the advantage that it allowsmovement of the substrate table and thus possibly also of theintermediate structure connected to it, as the area A may is such casebe large enough so that only a part of it provides the function ofclosing the liquid supply system and/or performing the measurements asdescribed below. Thus, flexibility may be offered thereby, as somemovements of the substrate table, e.g. in y direction, may be allowed asa size of the area A may be sufficiently large to cover such movements.This may allow some freedom of movement for the substrate table whenperforming activities in parallel by the lithographic apparatus, e.g.exchanging the substrate, swapping substrate tables in a dual stagelithographic apparatus etc., while at a same time offering immersionfluid supply closing functions and/or measuring functions by theintermediate structure as described here.

Also, the intermediate structure may comprise an immersion fluidfiltration device to filter the immersion fluid. The filtration devicemay comprise an inlet, a filter and an outlet, the inlet leading fromthe surface of the area A and the outlet leading towards the surface ofthe are A. The filter may e.g. be located in the intermediate structurebelow the area A, the inlet and outlet leading from the filter to thearea A. Further, it is possible that the surface of the intermediatestructure to close the fluid supply system, e.g. the area A, comprises asurface roughness which is substantially equal to a surface roughness ofthe substrate, thus providing substantially a same circulationresistance for a circulation of the immersion fluid, thus providing fora circulation of the immersion fluid which substantially equals acirculation of the fluid when the fluid supply system is closed by thesubstrate instead of by the area A, which therefore enables a highlyaccurate pressure measurement by the pressure measurement devicedescribed above, as a circulation of the fluid when closed by the area Asubstantially equals a circulation of the fluid when closed by thesubstrate.

The measurement system and intermediate structure may not only beapplied for a lithographic apparatus, but may be applied for anyposition apparatus in general, the positioning apparatus comprising asubstrate table constructed to hold a substrate, a reference structureand a measurement system to measure a position of the substrate tablewith respect to the reference structure, wherein the measurement systemcomprises a first measurement system to measure a position of thesubstrate table with respect to an intermediate structure, and a secondmeasurement system to measure a position of the intermediate structurewith respect to the reference structure. Preferred embodiments of thelithographic apparatus as described above may also be implemented withthe positioning apparatus as described here, providing same of similaradvantages and effects.

The intermediate structure, preferably the area A thereof, may, inaddition to or in stead of the features described above, furthercomprise one or more optical sensors 0, such as a dose sensor to measurea dose as projected by the projection system, an image sensor such as aTIS (Transmission Image Sensor) sensor to measure an image planeflatness or an aerial image as projected by the projection system,and/or an aberration sensor to measure an aberration of the projectionsystem. Measurements may be performed with these sensors when theintermediate structure has been positioned to e.g. hold the immersionfluid, thus the intermediate structure (and in particular the area Athereof) being positioned at or near a plane of focus of the projectionsystem. An advantage is that a higher throughput and a betterperformance of the lithographic apparatus may be provided thereby: inthe state of the art, optical sensors as described above may have beencomprised in the substrate table thus requiring an accurate positioningof the substrate table when performing measurements making use of theseoptical sensors. According to the embodiment described here, suchmeasurements may be performed while only the intermediate structure ispositioned to perform the measurements with the sensors, the substratetable may be used for other tasks at that moment in time (e.g.exchanging a wafer), thus allowing these tasks to be performed inparallel. During e.g. an exchanging or swapping of wafer stages afterexposures or whenever immersion fluid is held with the aid of theintermediate structure, parameters as the dose, the image plane flatnessand the lens aberrations may now be measured substantially withoutadditional time as no additional movements (e.g. to move a sensorassembly to a suitable location in or near the plane of focus) arerequired. Further, it is also possible that the sensors are movable withrespect to the intermediate structure, which may be advantageous in casethat the intermediate structure cannot move in x, y and/or z directionduring a swap of the substrate or during the fluid holding. In thiscase, the intermediate structure may comprise positioning sensors (e.g.interferometers, piezo etc) to identify (a position of) the movablesensor(s) comprised in the intermediate structure.

FIG. 6 shows a schematic side view of an embodiment of the substratetable WT, the intermediate structure IS, the fluid supply system LS tohold the immersion fluid IFL and a lens at the downstream end of theprojection system PS. The intermediate structure comprises an immersionfluid leakage drain IFLD to drain any immersion fluid which may leakbetween the substrate table WT and the intermediate structure IS whenapplying the intermediate structure as described with reference to FIG.5. The drained immersion liquid may be reused, e.g. after filtering itby a suitable filtration device. A leakage stopper LST may be providedat an edge of a surface of the intermediate structure which surface mayface the projection system, thus preventing a drain of immersion fluidat the edge.

Instead of or in addition to the first and second measurement systemdescribed above, the first and second measurement systems may alsocomprise (but not limited to) capacitive sensors, photonic sensors, eddycurrent sensors, magnetic encoders or any other sensor that canpreferably achieve (sub-) nm accuracy.

The measurement system of the lithographic apparatus and positioningapparatus as described here may not only be applied for measuring aposition of the substrate table, but may also be applied for measuring aposition of any movable part with respect to the reference structure,the first measurement system, thus measuring a position of the movablepart with respect to the intermediate structure. Thus, in the context ofthis document the wording ‘substrate table’ may also be understood as tocomprise any movable part of the lithographic apparatus and/or thepositioning apparatus.

Further, it is remarked that this invention is not limited to sensorheads on the intermediate structure and targets (e.g. mirrors andscales) on the substrate table and the reference structure respectively.One or more of the sensor heads may also be mounted on the substratetable (or reference frame) with the targets on the intermediatestructure.

Still further, for symmetry purposes a redundant interferometer (e.g.indicated as IF7) can be added having a same X and Z position asinterferometer IF 4 but having with regard to interferometer IF4 a sameY displacement as a distance between IF5 and IF6. This additionalinterferometer makes a metrology modeling easier and may improveaccuracy of the position measurements even further.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g. having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program stored therein.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1. A lithographic apparatus comprising: a stage comprising a substratetable constructed to hold a substrate; a reference structure; and ameasurement system to measure a position of the stage with respect tothe reference structure, wherein the measurement system comprises: afirst measurement system to measure a position of a part of the stagewith respect to an intermediate structure, and a second measurementsystem to measure a position of the intermediate structure with respectto the reference structure.
 2. A lithographic apparatus comprising: asubstrate table constructed to hold a substrate; a reference structure;and a measurement system to measure a position of the substrate tablewith respect to the reference structure, wherein the measurement systemcomprises: a first measurement system to measure a position of thesubstrate table with respect to an intermediate structure, and a secondmeasurement system to measure a position of the intermediate structurewith respect to the reference structure.
 3. A lithographic apparatuscomprising: a stage comprising a mirror block carrying a substrate tableconstructed to hold a substrate; a reference structure; and ameasurement system to measure a position of the mirror block withrespect to the reference structure, wherein the measurement systemcomprises: a first measurement system to measure a position of themirror block with respect to an intermediate structure, and a secondmeasurement system to measure a position of the intermediate structurewith respect to the reference structure.
 4. The lithographic apparatusaccording to one of the preceding claims, comprising a first drivemechanism to move the substrate table along a first axis, theintermediate structure being connectable to the first drive mechanism tofollow a movement of the substrate table along the first axis.
 5. Thelithographic apparatus according to claim 4, wherein the first axisextends along a surface of the reference structure, the first drivemechanism to move the intermediate structure substantially parallel tothe first axis.
 6. The lithographic apparatus according to claim 4,comprising a second drive mechanism to move the substrate table along asecond axis, the first and second axis defining a plane, the surface ofthe reference structure extending substantially parallel to the plane.7. The lithographic apparatus according to claim 6, wherein the seconddrive mechanism is arranged to move the substrate table with respect tothe first drive mechanism.
 8. The lithographic apparatus according toone of the claims 1-3, wherein the first measurement system in operationgenerates a plurality of first optical measurement beams between thesubstrate table and the intermediate structure, the first opticalmeasurement beams in a direction substantially parallel to the firstaxis.
 9. The lithographic apparatus according to claim 8, wherein thesecond measurement system in operation generates a plurality of secondoptical measurement beams between the intermediate structure and thereference structure, the second optical measurement beams in a directionperpendicular to the first axis.
 10. The lithographic apparatusaccording to claim 9, wherein the second optical measurement beams aresubstantially perpendicular to the plane defined by the first and secondaxis.
 11. The lithographic apparatus according to claim 9, wherein thesecond optical measurement beams are substantially parallel to thesecond axis.
 12. The lithographic apparatus according to one of theclaims 1-3, wherein the first measurement system comprises a first, atleast one second, and a third interferometer, and a first, a second anda third encoder.
 13. The lithographic apparatus according to claim 12,wherein the first and the at least one second interferometer areseparated from each other in a direction parallel to the second axis,and wherein the third interferometer is separated from the first andsecond interferometers in a direction perpendicular to the first andsecond axis.
 14. The lithographic apparatus according to claim 12,wherein the first and second encoder comprise a scale to measure aposition in a direction substantially perpendicular to the first andsecond axis, the third encoder comprising a scale to measure a positionin a direction substantially parallel to the second axis.
 15. Thelithographic apparatus according to claim 14, wherein the first andsecond encoder are separated from each other in the direction parallelto the second axis, the third encoder being separated from the first andsecond encoder in the direction perpendicular to the first and secondaxis.
 16. The lithographic apparatus according to one of the claims 1-3,wherein the second measurement system comprises a fourth, a fifth, and asixth interferometer, and a fourth, a fifth and a sixth encoder.
 17. Thelithographic apparatus according to claim 16, wherein the fourth and thefifth interferometer are separated from each other in the directionparallel to the second axis, and wherein the sixth interferometer isseparated from the fourth and fifth interferometers in a directionparallel to the first axis.
 18. The lithographic apparatus according toclaim 17, wherein the fifth and sixth encoder comprise a scale tomeasure a position in the direction substantially parallel to the firstaxis, the fourth encoder comprising a scale to measure a position in thedirection substantially parallel to the second axis.
 19. Thelithographic apparatus according to claim 18, wherein the fifth andsixth encoder are separated from each other in the direction parallel tothe second axis.
 20. The lithographic apparatus according to one of theclaims 1-3, wherein an extreme of the intermediate structure isconnected to the first drive mechanism by a leaf spring.
 21. Thelithographic apparatus according to one of the claims 1-3, wherein theintermediate structure is at least partly comprised of an invarmaterial.
 22. The lithographic apparatus according to one of the claims1-3, wherein the intermediate structure extends between the substratetable and the reference frame.
 23. The lithographic apparatus accordingto one of the claims 1-3, wherein lithographic apparatus is constructedto move the intermediate structure under an immersion hood of thelithographic apparatus when moving the substrate away from the immersionhood, a surface of the intermediate structure to close the immersionhood.
 24. The lithographic apparatus according to claim 23, comprising apressure measurement device to measure a pressure of an immersion fluidin the immersion hood, the pressure measurement device deriving apressure from a displacement of the intermediate structure when servingan the immersion hood closure plate.
 25. The lithographic apparatusaccording to claim 23, comprising the pressure measurement device tomeasure the pressure of the immersion fluid in the immersion hood, thepressure measurement device comprising a plurality of pressure sensorsarranged in the intermediate structure.
 26. The lithographic apparatusaccording to claim 23, wherein the intermediate structure comprises animmersion fluid filtration device.
 27. The lithographic apparatusaccording to claim 23, wherein the surface of the intermediate structureto close the immersion hood comprises surface roughness which issubstantially equal to a surface roughness of the substrate.
 28. Thelithographic apparatus according to one of the claims 1-3, wherein asurface of the intermediate structure facing the projection system,preferably the surface of the intermediate structure to close theimmersion hood, comprises an optical sensor comprising one or more of agroup of sensors comprising a dose sensor to measure a dose as projectedby the projection system, an image sensor to measure an image planeflatness or an aerial image as projected by the projection system,and/or an aberration sensor to measure an aberration of the projectionsystem.
 29. A positioning apparatus comprising: a stage comprising asubstrate table constructed to hold a substrate; a reference structure;and a measurement system to measure a position of the stage with respectto the reference structure, wherein the measurement system comprises: afirst measurement system to measure a position of a part of the stagewith respect to an intermediate structure, and a second measurementsystem to measure a position of the intermediate structure with respectto the reference structure.
 30. A positioning apparatus comprising: asubstrate table constructed to hold a substrate; a reference structure;and a measurement system to measure a position of the substrate tablewith respect to the reference structure, wherein the measurement systemcomprises: a first measurement system to measure a position of thesubstrate table with respect to an intermediate structure, and a secondmeasurement system to measure a position of the intermediate structurewith respect to the reference structure.
 31. The positioning apparatusaccording to claim 29 or 30, comprising a first drive mechanism to movethe substrate table along a first axis, the intermediate structure beingconnectable to the first drive mechanism to follow the movement of thesubstrate table along the first axis.
 32. The positioning apparatusaccording to claim 31, wherein the first axis extends along a surface ofthe reference structure, the first drive mechanism to move theintermediate structure substantially parallel to the first axis.
 33. Thepositioning apparatus according to claim 31, comprising a second drivemechanism to move the substrate table along a second axis, the first andsecond axis defining a plane, the surface of the reference structureextending substantially parallel to the plane.
 34. The positioningapparatus according to claim 33, wherein the second drive mechanism isarranged to move the substrate table with respect to the first drivemechanism.
 35. The positioning apparatus according to claim 29 or 30,wherein the first measurement system in operation generates a pluralityof first optical measurement beams between the substrate table and theintermediate structure, the first optical measurement beams in adirection substantially parallel to the first axis.
 36. The lithographicapparatus according to claim 35, wherein the second measurement systemin operation generates a plurality of second optical measurement beamsbetween the intermediate structure and the reference structure, thesecond optical measurement beams in a direction perpendicular to thefirst axis.